U.S. patent application number 15/079557 was filed with the patent office on 2016-09-29 for method for manufacturing alumina sintered body and alumina sintered body.
The applicant listed for this patent is NGK INSULATORS, LTD.. Invention is credited to Takuji KIMURA, Kazuhiro NOBORI.
Application Number | 20160280604 15/079557 |
Document ID | / |
Family ID | 56973921 |
Filed Date | 2016-09-29 |
United States Patent
Application |
20160280604 |
Kind Code |
A1 |
NOBORI; Kazuhiro ; et
al. |
September 29, 2016 |
METHOD FOR MANUFACTURING ALUMINA SINTERED BODY AND ALUMINA SINTERED
BODY
Abstract
A method for manufacturing an alumina sintered body, according
to the present invention, includes the steps of (a) obtaining a
compact by putting a slurry containing an Al.sub.2O.sub.3 powder, a
MgO powder, a MgF.sub.2 powder, a solvent, a dispersing agent, and
a gelatinizer into a mold, gelatinizing the slurry by a chemical
reaction of the gelatinizer in the mold, and causing mold release,
(b) obtaining a calcined body by drying the compact, performing
degreasing, and further performing calcination, and (c) obtaining a
ceramic sintered body by subjecting the calcined body to hot-press
firing at 1,150.degree. C. to 1,350.degree. C. In the step (a), the
Al.sub.2O.sub.3 powder having a purity of 99.9 percent by mass or
more is used and 0.1 to 0.2 parts by mass of MgO powder and 0.13
parts by mass or less of MgF.sub.2 powder relative to 100 parts by
mass of Al.sub.2O.sub.3 powder are used.
Inventors: |
NOBORI; Kazuhiro;
(Handa-City, JP) ; KIMURA; Takuji; (Kariya-City,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK INSULATORS, LTD. |
Nagoya-City |
|
JP |
|
|
Family ID: |
56973921 |
Appl. No.: |
15/079557 |
Filed: |
March 24, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 35/634 20130101;
C04B 2235/3217 20130101; C04B 2235/786 20130101; C04B 2235/96
20130101; C04B 35/62625 20130101; C04B 2235/77 20130101; C04B
35/62675 20130101; C04B 2235/5445 20130101; C04B 2235/3206
20130101; C04B 2235/784 20130101; C04B 2103/408 20130101; C04B
2235/6023 20130101; C04B 2237/343 20130101; C04B 35/62685 20130101;
C04B 2235/445 20130101; B32B 18/00 20130101; C04B 35/638 20130101;
C04B 2235/72 20130101; C04B 35/111 20130101 |
International
Class: |
C04B 35/10 20060101
C04B035/10; C04B 41/00 20060101 C04B041/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2015 |
JP |
2015-063671 |
Claims
1. A method for manufacturing an alumina sintered body comprising
the steps of: (a) obtaining a compact by putting a slurry
containing an Al.sub.2O.sub.3 powder, a MgO powder, a MgF.sub.2
powder, a solvent, a dispersing agent, and a gelatinizer into a
mold, gelatinizing the slurry by a chemical reaction of the
gelatinizer in the mold, and causing mold release; (b) obtaining a
calcined body by drying the compact, performing degreasing, and
further performing calcination; and (c) obtaining a ceramic
sintered body by subjecting the calcined body to hot-press firing
at 1,150.degree. C. to 1,350.degree. C., wherein in the step (a),
the Al.sub.2O.sub.3 powder having a purity of 99.9 percent by mass
or more is used and 0.1 to 0.2 parts by mass of the MgO powder and
0.13 parts by mass or less of the MgF.sub.2 powder relative to 100
parts by mass of the Al.sub.2O.sub.3 powder are used in preparation
of the slurry.
2. The method for manufacturing an alumina sintered body according
to claim 1, wherein in the step (a), 0.09 to 0.13 parts by mass of
the MgF.sub.2 powder relative to 100 parts by mass of the
Al.sub.2O.sub.3 powder are used in preparation of the slurry.
3. An alumina sintered body, containing 0.09 to 0.17 percent by
mass of Mg element and 0.03 to 0.04 percent by mass of F element,
and having a volume resistivity at 400.degree. C. of
1.0.times.10.sup.15 .OMEGA.cm or more, a withstand voltage of 130
kV/m or more, an average grain size of 0.5 to 10 .mu.m, and
3.sigma. of the grain size of 1 to 20 .mu.m, .sigma. representing a
standard deviation.
4. The alumina sintered body according to claim 3, having the
average grain size of 1 to 2 .mu.m and the 3.sigma. of the grain
size of 1 to 2 .mu.m.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method for manufacturing
an alumina sintered body and an alumina sintered body.
[0003] 2. Description of the Related Art
[0004] To date, electrostatic chucks may be used when semiconductor
wafers are subjected to surface treatments, e.g., film formation
and etching. An electrostatic chuck in which an electrostatic
electrode is buried in a disc-shaped alumina substrate and one
surface of the alumina substrate serves as a wafer-mounting surface
is known. An electrostatic force is generated between the
electrostatic electrode and the semiconductor wafer by energizing
the electrostatic electrode while the semiconductor wafer is
mounted on the wafer-mounting surface of the electrostatic chuck.
The semiconductor wafer is attracted and held by the wafer-mounting
surface on the basis of the electrostatic force. Also, a heater
electrode may be buried in the alumina substrate together with the
electrostatic electrode. In this case, when the heater electrode is
energized while the semiconductor wafer is mounted on the
wafer-mounting surface, the semiconductor wafer is heated by the
heater electrode. As for the method for manufacturing the alumina
substrate used for such an electrostatic chuck, a method described
in PTL 1 is known. According to PTL 1, initially, a slurry
containing an alumina powder having a purity of 99.7%, a small
amount of MgO raw material powder, a binder, water, and a
dispersing agent is produced. Subsequently, the slurry is
spray-dried, and the binder is removed at 500.degree. C. so as to
produce alumina granules. The resulting alumina granules are filled
into a mold, and press forming is performed to produce a compact.
The resulting compact is set into a carbon sheath and hot-press
firing is performed so as to produce an alumina sintered body. The
hot-press firing is performed by holding the compact in a nitrogen
atmosphere (150 kPa) at 1,600.degree. C. for 2 hours while a
pressure of 100 kg/cm.sup.2 is applied, for example. The thus
obtained alumina sintered body is subjected to grinding so as to
produce the alumina substrate.
CITATION LIST
Patent Literature
[0005] PTL 1: JP 2008-53316 A
SUMMARY OF THE INVENTION
[0006] However, in the prior art electrostatic chuck described
above, there is a problem that variations in release response at
high temperatures are large. It is known that the release response
correlates with the volume resistivity and the withstand voltage.
Therefore, development of an alumina sintered body having volume
resistivity at high temperatures and withstand voltage higher than
ever and exhibiting small lot-to-lot variations in volume
resistivity and withstand voltage has been desired.
[0007] The present invention was made in order to solve such issues
and a main object is to provide an alumina sintered body having
high volume resistivity at high temperatures and high withstand
voltage and exhibiting small lot-to-lot variations in volume
resistivity and withstand voltage.
[0008] A method for manufacturing an alumina sintered body,
according to the present invention, includes the steps of:
[0009] (a) obtaining a compact by putting a slurry containing an
Al.sub.2O.sub.3 powder, a MgO powder, a MgF.sub.2 powder, a
solvent, a dispersing agent, and a gelatinizer into a mold,
gelatinizing the slurry by a chemical reaction of the gelatinizer
in the mold, and causing mold release,
[0010] (b) obtaining a calcined body by drying the compact,
performing degreasing, and further performing calcination, and
[0011] (c) obtaining a ceramic sintered body by subjecting the
calcined body to hot-press firing at 1,150.degree. C. to
1,350.degree. C.,
[0012] wherein in the step (a), the Al.sub.2O.sub.3 powder having a
purity of 99.9 percent by mass or more is used and 0.1 to 0.2 parts
by mass of the MgO powder and 0.13 parts by mass or less of the
MgF.sub.2 powder relative to 100 parts by mass of the
Al.sub.2O.sub.3 powder are used in preparation of the slurry.
[0013] According to the method for manufacturing an alumina
sintered body, it is possible to obtain an alumina sintered body
having high volume resistivity at high temperatures and high
withstand voltage and exhibiting small lot-to-lot variations in
volume resistivity and withstand voltage. Such an alumina sintered
body is obtained because the amounts of addition of the MgF.sub.2
powder, which functions as a sintering aid, and the MgO powder,
which functions as a grain growth inhibitor, to the slurry are
within appropriate ranges. If the amount of the MgF.sub.2 powder is
too large, the grain size becomes too large and degradation of the
characteristics is caused. If the amount of the MgF.sub.2 powder is
too small, sintering does not proceed easily. If the amount of the
MgO powder is too small, the grain size becomes too large and
degradation of the characteristics is caused. If the amounts of the
MgO powder and the MgF.sub.2 powder are too large, different phases
(MgAl.sub.2O.sub.4 and the like) having relatively high electrical
conductivity are generated in alumina and, thereby, the volume
resistivity decreases. Meanwhile, the volume resistivity at high
temperatures and the withstand voltage of the alumina sintered body
obtained by this method are stable irrespective of lot. The
characteristics are stable because a so-called gel casting method
(method in which forming of slurry is performed by gelatinization)
is adopted, so that the Al.sub.2O.sub.3 powder, the MgF.sub.2
powder, and the MgO powder are easily homogeneously dispersed.
[0014] In this regard, in consideration of the release response at
high temperatures, it is preferable that the volume resistivity at
400.degree. C. of the alumina sintered body be 1.0.times.10.sup.15
.OMEGA.cm or more and the withstand voltage be 130 kV/mm or more.
The average grain size is preferably 0.5 to 10 .mu.m. It is
preferable that 3.sigma. (.sigma. represents a standard deviation)
of the grain size be 1 to 20 .mu.m. It is preferable that the
composition contain 0.09 to 0.17 percent by mass of Mg element and
0.03 to 0.04 percent by mass of F element. The manufacturing method
according to the present invention is suitable for production of
such an alumina sintered body.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a sectional view of a member 1 for a semiconductor
production apparatus.
[0016] FIGS. 2A to 2D are step diagrams showing a procedure for
producing an electrostatic chuck 10.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The preferred embodiment according to the present invention
will be described below with reference to the drawings. FIG. 1 is a
sectional view of a member 1 for a semiconductor production
apparatus.
[0018] The member 1 for a semiconductor production apparatus
includes an electrostatic chuck 10, which can attract a silicon
wafer W to be subjected to a plasma treatment, and a cooling plate
18 serving as a support disposed on the back surface of the
electrostatic chuck 10.
[0019] The electrostatic chuck 10 includes a disc-shaped alumina
substrate 12 and a heater electrode 14 and an electrostatic
electrode 16 which are buried in the alumina substrate 12. The
upper surface of the alumina substrate 12 is a wafer-mounting
surface 12a. The heater electrode 14 is, for example, patterned
unicursally in such a way that wiring can be performed on the
entire surface of the alumina substrate 12, and generates heat by
application of a voltage so as to heat the wafer W. The heater
electrode 14 is a mixture of molybdenum and alumina. A voltage can
be applied to the heater electrode 14 by bar terminals (not shown
in the drawing) which reach one end and the other end of the heater
electrode 14 from the back of the cooling plate 18. The
electrostatic electrode 16 is a planar electrode, which can be
applied with a direct current voltage by an external power supply
not shown in the drawing. When a direct current voltage is applied
to the electrostatic electrode 16, the wafer W is attracted and
fixed to the wafer-mounting surface 12a by a Coulomb force or
Johnson-Rahbek force. When the application of the direct current
voltage is stopped, the attraction and fixing of the wafer W to the
wafer-mounting surface 12a are released.
[0020] The cooling plate 18 is a metal (for example, aluminum) disc
member and is bonded to the surface opposite to the wafer-mounting
surface 12a of the electrostatic chuck 10 with a bonding layer,
although not shown in the drawing, therebetween. The cooling plate
18 has a coolant channel 20, through which a coolant (for example,
water) cooled by an external cooler, although not shown in the
drawing, is circulated. For example, the coolant channel 20 is
formed unicursally in such a way that the coolant passes throughout
the cooling plate 18.
[0021] Next, an application example of the thus configured member 1
for a semiconductor production apparatus will be described. The
member 1 for a semiconductor production apparatus is disposed in a
chamber, although not shown in the drawing, and is used for etching
the surface of the wafer W by the plasma generated in the chamber.
At this time, the temperature of the wafer W is controlled to be
constant by adjusting the amount of electric power supplied to the
heater electrode 14 or adjusting the flow rate of the coolant
circulated through the coolant channel 20 in the cooling plate
18.
[0022] Next, a procedure for producing the electrostatic chuck 10
constituting the member 1 for a semiconductor production apparatus
will be described. In the following description, a procedure for
producing the alumina substrate 12 will be explained in addition to
the explanation of the procedure for producing the electrostatic
chuck 10.
[0023] 1. Production of Compact (Refer to FIG. 2A, an Example of
the Step (a) According to the Present Invention)
[0024] First to third compacts 51 to 53 are produced. The compacts
51 to 53 are produced by putting a slurry containing an
Al.sub.2O.sub.3 powder, a MgO powder serving as a grain growth
inhibitor, a MgF.sub.2 powder serving as a sintering aid, a
solvent, a dispersing agent, and a gelatinizer into each of molds
(first to third molds), gelatinizing the slurry by a chemical
reaction of the gelatinizer in the mold, and causing mold
release.
[0025] There is no particular limitation regarding what material
may be used for the solvent as long as the dispersing agent and the
gelatinizer are dissolved. Examples of the solvent include
hydrocarbon solvents (toluene, xylene, solvent naphtha, and the
like), ether solvents (ethylene glycol monoethyl ether, butyl
carbitol, butyl carbitol acetate, and the like), alcohol solvents
(isopropanol, 1-butanol, ethanol, 2-ethylhexanol, terpineol,
ethylene glycol, glycerin, and the like), ketone solvents (acetone,
methyl ethyl ketone, and the like), ester solvents (butyl acetate,
dimethyl glutarate, triacetin, and the like), and polybasic acid
solvents (glutaric acid and the like). In particular, it is
preferable that solvents having at least two ester bonds, e.g.,
polybasic acid esters (dimethyl glutarate and the like) and acid
esters of polyhydric alcohols (triacetin and the like), be
used.
[0026] There is no particular limitation regarding what material
may be used for the dispersing agent as long as the Al.sub.2O.sub.3
powder is homogeneously dispersed into the solvent. Examples of the
dispersing agent include polycarboxylic acid copolymers,
polycarboxylates, sorbitan fatty acid esters, polyglycerol fatty
acid esters, phosphate ester salt copolymers, sulfonate copolymers,
and polyurethane polyester copolymers having tertiary amines. In
particular, it is preferable that polycarboxylic acid copolymers
and polycarboxylates be used. The dispersing agent is added and,
thereby, the slurry before forming can have low viscosity and, in
addition, high fluidity.
[0027] The gelatinizer may contain, for example, isocyanates,
polyols, and a catalyst. There is no particular limitation
regarding what material may be used for the isocyanates, among
them, as long as the material has an isocyanate group as a
functional group. Examples of the isocyanates include tolylene
diisocyanate (TDI), diphenylmethane diisocyanate (MDI), and
modified products thereof. In this regard, a reactive functional
group other than the isocyanate group may be contained in the
molecule, and further, a large number of reactive functional groups
may be contained, as in polyisocyanates. There is no particular
limitation regarding what material may be used for the polyols as
long as the material has at least two hydroxyl groups which can
react with isocyanate groups. Examples of the polyols include
ethylene glycol (EG), polyethylene glycol (PEG), propylene glycol
(PG), polypropylene glycol (PPG), polytetramethylene glycol (PTMG),
polyhexamethylene glycol (PHMG), and polyvinyl alcohol (PVA). There
is no particular limitation regarding what material may be used for
the catalyst as long as the catalyst facilitates an urethane
reaction between the isocyanates and the polyols. Examples of the
catalyst include triethylenediamine, hexanediamine, and
6-dimethylamino-1-hexanol.
[0028] In this step, preferably, the solvent and the dispersing
agent are added to the Al.sub.2O.sub.3 powder, the MgO powder, and
the MgF.sub.2 powder at a predetermined ratio, these are mixed for
a predetermined time so as to prepare a slurry precursor, the
gelatinizer is added to the resulting slurry precursor, and mixing
and vacuum degassing are performed so as to produce the slurry. The
Al.sub.2O.sub.3 powder having a purity of 99.9 percent by mass or
more is used and 0.1 to 0.2 parts by mass of MgO powder and 0.13
parts by mass or less (preferably 0.09 to 0.13 parts by mass) of
MgF.sub.2 powder relative to 100 parts by mass of Al.sub.2O.sub.3
powder are used. There is no particular limitation regarding what
mixing method may be used for preparing the slurry precursor and
the slurry. For example, ball milling, rotary and revolutionary
agitation, vibratory agitation, propeller agitation, and the like
can be used. In this regard, preferably, the slurry, in which the
gelatinizer is added to the slurry precursor, is poured into the
mold promptly because the chemical reaction (urethane reaction) of
the gelatinizer starts with a lapse of time. The slurry poured into
the mold gels by the chemical reaction of the gelatinizer contained
in the slurry. The chemical reaction of the gelatinizer refers to a
reaction in which the isocyanates and the polyols are converted to
urethane resins (polyurethanes) by the urethane reaction. The
slurry gels by the reaction of the gelatinizer and the urethane
resin functions as an organic binder.
[0029] 2. Production of Calcined Body (Refer to FIG. 2B, an Example
of the Step (b) According to the Present Invention)
[0030] After the first to third compacts 51 to 53 are dried,
degreasing and, in addition, calcination are performed so as to
obtain first to third calcined bodies 61 to 63. The drying of the
compacts 51 to 53 is performed in order to vaporize the solvent
contained in the compacts 51 to 53. The drying temperature and the
drying time may be set appropriately in accordance with the solvent
used. However, the drying temperature is set with attention not to
cause cracking in the compacts 51 to 53 during drying. Meanwhile,
the atmosphere may be any one of an air atmosphere, an inert
atmosphere, and a vacuum atmosphere. Degreasing of the compacts 51
to 53 after drying is performed in order to decompose and remove
organic materials, e.g., the dispersing agent, the catalyst, and
the binder. The degreasing temperature may be set appropriately in
accordance with the type of the contained organic material and may
be set at 400.degree. C. to 600.degree. C. The atmosphere may be
any one of an air atmosphere, an inert atmosphere, and a vacuum
atmosphere. Calcination of the compacts 51 to 53 after degreasing
is performed in order to enhance the strength and facilitate
handling. The calcination temperature is not specifically limited
but may be set at, for example, 750.degree. C. to 900.degree. C.
The atmosphere may be any one of an air atmosphere, an inert
atmosphere, and a vacuum atmosphere.
[0031] 3. Printing of Electrode Paste (Refer to FIG. 2C)
[0032] One surface of the first calcined body 61 is printed with a
heater electrode paste 71 in a predetermined heater electrode
pattern, and one surface of the third calcined body 63 is printed
with an electrostatic electrode paste 72 in a predetermined
electrostatic electrode pattern. Both the pastes 71 and 72 contain
an alumina powder and a molybdenum powder. Examples of the binder
include cellulose binders (ethyl cellulose and the like), acrylic
binders (polymethyl methacrylate and the like), and vinyl binders
(polyvinyl butyral and the like). Examples of the solvent include
terpineol. Examples of the printing method include a screen
printing method.
[0033] 4. Hot-Press Firing (Refer to FIG. 2D, an Example of the
Step (c) According to the Present Invention)
[0034] The first calcined body 61 and the second calcined body 62
are stacked with the printed heater electrode paste 71 therebetween
and, in addition, the second calcined body 62 and the third
calcined body 63 are stacked with the printed electrostatic
electrode paste 72 therebetween. Hot-press firing is performed in
that state. Consequently, the heater electrode paste 71 is made
into the heater electrode 14 by firing, the electrostatic electrode
paste 72 is made into the electrostatic electrode 16 by firing, and
the individual calcined bodies 61 to 63 are integrated into the
alumina substrate 12 by sintering, so that the electrostatic chuck
10 is obtained. In the hot-press firing, the press pressure is
specified to be preferably 30 to 300 kgf/cm.sup.2 at least at a
maximum temperature (firing temperature), and more preferably 50 to
250 kgf/cm.sup.2. In this regard, the maximum temperature may be
set at a low temperature (1,150.degree. C. to 1,350.degree. C.) as
compared with the case where the MgF.sub.2 powder is not added
because the MgF.sub.2 powder serving as the sintering aid is added
to the Al.sub.2O.sub.3 powder. The atmosphere may be selected
appropriately among an air atmosphere, an inert atmosphere, and a
vacuum atmosphere. In this regard, it is not desirable to set the
maximum temperature at lower than 1,150.degree. C. because
sintering of the Al.sub.2O.sub.3 powder does not proceed
sufficiently. It is not desirable to set the maximum temperature at
higher than 1,350.degree. C. because alumina sintered grains become
excessively large and various characteristics are degraded.
[0035] In the present embodiment described in detail above, the
resulting alumina substrate 12 has high volume resistivity at high
temperatures and high withstand voltage. Such an alumina substrate
12 is obtained because the amounts of addition of the MgF.sub.2
powder which functions as the sintering aid and the MgO powder
which functions as the grain growth inhibitor in the slurry are
within appropriate ranges. Also, the volume resistivity at high
temperatures and the withstand voltage of the thus obtained alumina
substrate 12 are stable irrespective of lot. The characteristics
are stable because a so-called gel casting method (method in which
forming of slurry is performed by gelatinization) is adopted, so
that the Al.sub.2O.sub.3 powder, the MgO powder, and the MgF.sub.2
powder are easily homogeneously dispersed.
[0036] In consideration of the release response at high
temperatures, preferably, the volume resistivity at 400.degree. C.
of the alumina sintered body 12 is 1.0.times.10.sup.15 .OMEGA.cm or
more and the withstand voltage is 130 kV/mm or more. The average
grain size is preferably 0.5 to 10 .mu.m. It is preferable that
3.sigma. (.sigma. represents a standard deviation) of the grain
size be 1 to 20 .mu.m. It is preferable that the composition
contain 0.09 to 0.17 percent by mass of Mg element and 0.03 to 0.04
percent by mass of F element. The above-described manufacturing
method is suitable for production of such an alumina sintered body
12.
[0037] In this regard, needless to say, the present invention is
not limited to the above-described embodiment and can be executed
in various forms within the technical scope of the present
invention.
[0038] For example, in the above-described embodiment, the
electrostatic chuck 10 in which the heater electrode 14 and the
electrostatic electrode 16 are buried in the alumina substrate 12
is shown. However, only one of the heater electrode 14 and the
electrostatic electrode 16 may be buried in the alumina substrate
12, or a high frequency electrode for plasma generation may be
buried in place of or in addition to them.
[0039] In the above-described embodiment, one surface of the first
calcined body 61 is printed with the heater electrode paste 71, one
surface of the third calcined body 63 is printed with the
electrostatic electrode paste 72 and, thereafter, the first to
third calcined bodies 61 to 63 in the stacked state are subjected
to hot-press firing. However, the following procedure may be
adopted. That is, one surface of the first compact 51 is printed
with the heater electrode paste, and one surface of the third
compact 53 is printed with the electrostatic electrode paste 72.
Subsequently, the first to third compacts 51 to 53 are dried,
degreased, and further calcined so as to produce the first to third
calcined bodies. Thereafter, these may be stacked and subjected to
hot-press firing.
[0040] In the above-described embodiment, an example of the method
for manufacturing the electrostatic chuck 10 is shown. However, the
method may applied to a method for manufacturing an alumina
substrate with no built-in electrode, where the heater electrode 14
and the electrostatic electrode 16 are not included. In that case,
three calcined bodies may be stacked and subjected to hot-press
firing, or only one compact may be subjected to drying, degreasing,
and calcination and, thereafter, may be subjected to hot-press
firing so as to produce the alumina substrate.
Examples
[0041] The examples according to the present invention will be
described below. In this regard, the following examples do not
limit the present invention.
Experimental Example 1
[0042] An electrostatic chuck 10 was produced as described below in
conformity with the above-described production procedure (refer to
FIGS. 2A-2D).
(a) Production of Compact
[0043] A slurry precursor was prepared by weighing 100 parts by
mass of Al.sub.2O.sub.3 powder (average grain size of 0.5 .mu.m and
purity of 99.99%), 0.1 parts by mass of MgO powder, 0.09 parts by
mass of MgF.sub.2 powder, 3 parts by mass of polycarboxylic acid
copolymer serving as the dispersing agent, and 20 parts by mass of
polybasic acid ester serving as the solvent and mixing them for 14
hours with a ball mill (trommel). A slurry was obtained by adding
the gelatinizer, that is, 3.3 parts by mass of 4,4'-diphenylmethane
diisocyanate as the isocyanates, 0.3 parts by mass of ethylene
glycol as the polyols, and 0.1 parts by mass of
6-dimethylamino-1-hexanol as the catalyst, to the resulting slurry
precursor and performing mixing for 12 minutes with a rotary and
revolutionary agitator. The resulting slurry was poured into each
of the first to third molds used in the above-described embodiment.
Thereafter, standing at 22.degree. C. for 2 hours was performed.
The slurry was gelatinized on the basis of a chemical reaction of
the gelatinizer in each mold and was released from the mold. In
this manner, the first to third compacts 51 to 53 (refer to FIG.
2A) were obtained from the first to third molds, respectively.
(b) Production of Calcined Body
[0044] The first to third calcined bodies 61 to 63 (refer to FIG.
2B) were obtained by drying the first to third compacts 51 to 53 at
100.degree. C. for 10 hours, performing degreasing at a maximum
temperature of 550.degree. C. for 1 hour in an air atmosphere, and
further performing calcination at a maximum temperature of
820.degree. C. for 1 hour in an air atmosphere.
(c) Printing of Electrode Paste
[0045] An electrode paste was produced by preparing a molybdenum
powder and an Al.sub.2O.sub.3 powder in such a way that the alumina
content became 10 percent by mass, adding polymethylmethacrylate
serving as the binder and terpineol serving as the solvent, and
performing mixing. The resulting electrode paste was used for both
the electrostatic electrode and the heater electrode. Then, one
surface of the first calcined body 61 was screen-printed with the
heater electrode paste 71 and one surface of the third calcined
body 63 was screen-printed with the electrostatic electrode paste
72 (refer to FIG. 2C).
(d) Hot-Press Firing
[0046] The first and second calcined bodies 61 and 62 were stacked
with the heater electrode paste 71 therebetween and, in addition,
the second and third calcined bodies 62 and 63 were stacked with
the electrostatic electrode paste 72 therebetween. Subsequently,
hot-press firing was performed in that state. Consequently, the
heater electrode paste 71 was made into the heater electrode 14 by
firing, the electrostatic electrode paste 72 was made into the
electrostatic electrode 16 by firing, and the individual calcined
bodies 61 to 63 were integrated into the alumina substrate 12 by
sintering (refer to FIG. 2D). The hot-press firing was performed by
conducting holding for 2 hours in a vacuum atmosphere at a pressure
of 250 kgf/cm.sup.2 and a maximum temperature of 1,260.degree. C.
Thereafter, the ceramic sintered body surface was subjected to
surface grinding with a diamond wheel, so that the thickness from
the electrostatic electrode 16 to the wafer-mounting surface 12a
was specified to be 350 .mu.m and the thickness from the heater
electrode 14 to the other surface was specified to be 750 .mu.m.
Subsequently, side surface processing and perforation ware
performed and terminals were attached, so that the electrostatic
chuck 10 with built-in heater electrode 14 and electrostatic
electrode 16 and a diameter of 300 mm was obtained.
[0047] The following characteristics of the alumina substrate 12 in
the resulting electrostatic chuck 10 were measured. The results
thereof are shown in Table 1. In this regard, as for sane
characteristics, aluminum test pieces (without electrode) were
produced separately in the same production procedure as in
Experimental example 1.
[0048] Relative Density
[0049] It is assumed that all the individual raw materials
(Al.sub.2O.sub.3, MgO, and MgF.sub.2) mixed at the time of the
production remain in the alumina sintered body while keeping their
original forms, and the theoretical density of the sintered body is
determined from the theoretical densities of the individual raw
materials and the amounts of use (parts by mass) of the individual
raw materials. Thereafter, the bulk density determined by the
Archimedes method is divided by the theoretical density of the
sintered body, the quotient is multiplied by 100, and the resulting
value is taken as the relative density (%) of the sintered body.
Therefore, if the amounts of use of the individual raw materials
are not changed, the relative density increases as the bulk density
increases.
[0050] Strength
[0051] A four-point bending test was performed in conformity with
JIS R1601, and the strength was calculated.
[0052] Average Grain Size and 3.sigma.
[0053] A linear analysis was employed for determination.
Specifically, a fracture of each sintered body was observed by an
electron microscope. Arbitrary number of lines were drawn on the
resulting SEM photograph and an average length of the intercept was
determined. The accuracy was improved as the number of grains
across the lines increased. Therefore, the number of lines drawn
was determined in such a way that the lines crossed about 60
grains, although the number of lines was different depending on the
grain size. The average grain size was estimated from the average
length of the intercept. Also, the standard deviation .sigma. was
determined from the individual grain sizes used for estimating the
average grain size, and the resulting .sigma. was multiplied by 3
so as to determine 3.sigma..
[0054] Volume Resistivity
[0055] The measurement was performed in an air atmosphere at
400.degree. C. by a method in conformity with JIS C2141. The test
piece shape was specified to be 50 mm square.times.0.2 mm thick and
the individual electrodes were made of silver in such a way that a
main electrode had a diameter of 20 mm, a guard electrode had an
inner diameter of 30 mm and an outer diameter of 40 mm, and an
application electrode had a diameter of 40 mm. The applied voltage
was specified to be 2 kV/mm, a current value 30 minutes after
application of the voltage was read, and the volume resistivity was
calculated from the resulting current value. Also, five test pieces
of Experimental example 1 were prepared and the standard deviation
of the volume resistivity at 400.degree. C. thereof was
determined.
[0056] Withstand Voltage
[0057] The test piece shape was specified to be 50 mm
square.times.0.2 mm thick. The withstand voltage of the test piece
was measured in conformity with JIS C2110. Also, five test pieces
of Experimental example 1 were prepared and the standard deviation
of the withstand voltage thereof was determined.
[0058] Chemical Analysis
[0059] The Mg content was determined by inductively coupled plasma
(ICP) emission spectrometry. In this regard, the lower limit of the
measurement of the Mg content was 1 ppm. Meanwhile, the F content
was determined by pyrohydrolysis separation-ion chromatography (JIS
R9301-3-11). In this regard, the lower limit of the measurement of
the F content was 10 ppm.
TABLE-US-00001 TABLE 1 Raw Material Powder Al.sub.2O.sub.3
MgO*.sup.1 MgF.sub.2*.sup.2 CaO*.sup.3 Producion Condition
Experimental Purity Ratio by Ratio by Ratio by Forming Degreasing
Calcination Firing*.sup.4 Example No. (%) Mass Mass Mass Method
(.degree. C.) (.degree. C.) (.degree. C.) 1 99.99 0.1 0.09 0 Gel
550 820 1260 casting 2 99.99 0.1 0.13 0 Gel 550 820 1260 casting 3
99.99 0.1 0.2 0 Gel 550 820 1260 casting 4 99.99 0.1 0.3 0 Gel 550
820 1260 casting 5 99.99 0.2 0.09 0 Gel 550 820 1260 casting 6
99.99 0.2 0.13 0 Gel 550 820 1260 casting 7 99.99 0.2 0.2 0 Gel 550
820 1260 casting 9 99.99 0.2 0.3 0 Gel 550 820 1260 casting 9 99.99
0.3 0.09 0 Gel 550 820 1260 casting 10 99.5 0.05 0 0.03 Press 450
1620 forming Alumina Substrate (Sintered Body) Chemical Volume
Withstand Experimental Analysis Relative Strength Grain Size
Resistivity Voltage Example No. Mg (%) F (%) Density (MPa) Average
3.sigma. (.OMEGA.cm)*.sup.5 (kV/mm) 1 0.09 0.03 0.998 380 1.5 --
5.1E+15 150 [0.12]*.sup.6 [0.03]*.sup.7 2 0.11 0.04 0.998 350 1.7
-- 2.0E+15 141 [0.14] [0.03] 3 0.14 0.06 0.998 340 10.0 -- 8.0E+14
135 [0.21] [0.03] 4 0.18 0.09 0.998 300 18.0 -- 6.0E+14 110 [0.35]
[0.04] 5 0.15 0.03 0.998 380 1.0 1.5 3.4E+15 160 [0.16] [0.02] 6
0.17 0.04 0.998 350 1.6 1.9 1.2E+15 138 [0.17] [0.03] 7 0.20 0.06
0.998 320 8.0 15.2 7.0E+14 130 [0.25] [0.03] 9 0.23 0.09 0.998 300
15.0 22.2 6.0E+14 120 [0.39] [0.04] 9 0.21 0.03 0.996 390 1.0 --
5.5E+14 95 [0.73] [0.05] 10 0.03 0 0.996 315 16.6 30.1 1.0E+14 86
[1.9] [0.08] *.sup.1Ratio of parts by weight of MgO to 100 parts by
weight of Al.sub.2O.sub.3. *.sup.2Ratio of parts by weight of
MgF.sub.2 to 100 parts by weight of Al.sub.2O.sub.3. *.sup.3Ratio
of parts by weight of CaO to 100 parts by weight of
Al.sub.2O.sub.3. *.sup.4The temperature is the maximum temperature
during firing. *.sup.5Volume resistivety at 400.degree. C.
*.sup.6Value in square brackets [ ] is standard deviation (N = 5)
of the volume resistivity at 400.degree. C. *.sup.7Value in square
brackets [ ] is standard deviation (N = 5) of withstand
voltage.
Experimental Examples 2 to 9
[0060] In Experimental examples 2 to 9, electrostatic chucks 10
were produced in the same manner as in Experimental example 1
except that the parts by mass of the MgO powder and the MgF.sub.2
powder relative to 100 parts by mass of the Al.sub.2O.sub.3 powder
were changed as shown in Table 1. Also, the characteristics of the
alumina substrate 12 of each of Experimental examples 2 to 9 were
measured in the same manner as in Experimental example 1. The
results thereof are shown in Table 1.
Experimental Example 10
[0061] In Experimental example 10, an alumina substrate, in which
an electrode was not buried, was produced. Initially, an
Al.sub.2O.sub.3 powder having a purity of 99.5%, a MgO powder, and
a CaO powder were mixed in such a way that the MgO powder was 0.05
parts by mass and the CaO powder was 0.03 parts by mass relative to
100 parts by mass of alumina. Polyvinyl alcohol (PVA) serving as a
binder, water, and a dispersing agent were added to the resulting
mixed powder, and mixing was performed for 16 hours with a trammel
so as to produce a slurry. The resulting slurry was spray-dried by
using a spray drier. Thereafter, the binder was removed by
conducting holding at 450.degree. C. for 5 hours so as to produce
alumina granules of about 80 .mu.m in average grain size. The
resulting alumina granules were filled into a mold, and press
forming was performed at a pressure of 200 kg/cm.sup.2 so as to
produce a compact. Subsequently, the resulting compact was set into
a carbon sheath and was fired by using a hot-press firing method.
The firing was performed in a pressurized nitrogen atmosphere (150
kPa) by raising a temperature at 300.degree. C./h and conducting
holding at 1,620.degree. C. for 2 hours while a pressure of 100
kg/art was applied so as to produce an alumina sintered body. The
thus obtained alumina sintered body was subjected to grinding so as
to produce an alumina substrate having a diameter of 300 mm and a
thickness of 6 mm. Various characteristics of the alumina substrate
of Experimental example 10 were measured in the same manner as in
experimental example 1. The results thereof are shown in Table
1.
[0062] Evaluation
[0063] In Experimental examples 1, 2, 5, and 6, alumina substrates
having high volume resistivity of 1.0.times.10.sup.15 .OMEGA.cm or
more at 400.degree. C. and high withstand voltage of 130 kV/mm or
more were obtained. Meanwhile, in the case where a plurality of
test pieces of each of Experimental examples 1, 2, 5, and 6 were
produced, the standard deviations of these characteristics were
small values and, therefore, it was found that lot-to-lot
variations were small. The average grain size of the alumina
sintered grains was 1 to 2 .mu.m, and 3.sigma. (.sigma. is standard
deviation) of the grain size was 1 to 2 .mu.m. In these
experimental examples, the Al.sub.2O.sub.3 powder having a purity
of 99.9% or more was used, and 0.1 to 0.2 parts by mass of MgO
powder and 0.13 parts by mass or less of MgF.sub.2 powder relative
to 100 parts by mass of Al.sub.2O.sub.3 powder were used.
[0064] On the other hand, in each of Experimental examples 3, 4, 7,
and 8, the amount of use of the MgF.sub.2 powder serving as the
sintering aid was large, so that grain growth proceeded
excessively, and the average grain size of the alumina sintered
grains was 8 .mu.m or more and was too large. Therefore, the volume
resistivity at 400.degree. C. and the withstand voltage were
reduced. In addition, lot-to-lot variations increased.
[0065] In Experimental example 9, the amount of use of the MgO
serving as the grain growth inhibitor was large. Therefore, the
average grain size of the alumina sintered grains was small, but
the relative density did not increase, so that the volume
resistivity at 400.degree. C. and the withstand voltage were
reduced. In addition, lot-to-lot variations increased.
[0066] In Experimental example 10, press forming rather than the
gel casting method was adopted as the forming method and firing was
performed at a high temperature. Therefore, the volume resistivity
at 400.degree. C. and the withstand voltage were reduced. In
addition, lot-to-lot variations increased. The average grain size
of the alumina sintered grains was more than 15 .mu.m and 3.sigma.
was large. Consequently, it was found that the range of the grain
size distribution increased.
[0067] Experimental examples 1, 2, 5, and 6 correspond to the
examples according to the present invention, and the other
Experimental examples correspond to the comparative examples.
[0068] The present application claims priority from Japanese Patent
Application No. 2015-063671 filed on Mar. 26, 2015, the entire
contents of which are incorporated herein by reference.
* * * * *